There has been considerable interest in recent years in the development of a monitoring system for obtaining a continuous measurement of a patient's blood pressure. One of the most promising techniques for obtaining such a continuous measurement involves the use of an arterial tonometer comprising an array of small pressure sensing elements fabricated in a silicon "chip." The use of such an array of sensor elements for blood pressure measurements is disclosed generally in U.S. Pat. No. 4,269,193 issued to Newgard and U.S. Pat. No. 4,423,738 issued to Eckerle.
In general, tonometric blood pressure measurement techniques require placement of the sensor over a superficial artery with a sufficient hold-down force to partially flatten the artery between opposing faces of the sensor and an underlying bone. In the past, the sensor was typically contained in a housing strapped to the patient's wrist such that the radial artery was beneath the sensor. At the point on the wrist where the sensor was normally attached, the radial artery passes over the radius bone and is next to the tendon of the flexor carpi radialis muscle. In prior designs, the sensor was glued to the bottom of a lead frame and its surface was flush with the bottom of a rigid metal frame, to which a flexible bellows was attached. The required hold down force was provided by internal pressure in the sensor housing above the bellows. Because the lower rim of the rigid metal frame typically rested on the radius bone and the flexor tendon, a hold down force corresponding to a pressure of greater than 200 mm of mercury inside the housing often was required to properly flatten the artery between the sensor and the underlying bone. Pressures of this magnitude can cause discomfort for the patient, especially when the measurement is taken for an extended period of time.